Why do I lose TV channels on freeview

[akelfor]

Every couple of days I completely lose all ITV channels including ITV1,2,3,4 and Ch 4. The digital tv shows channel as either "not available" or "not tuned". At first I tried to auto update the channel tuning but this had no effect. The channels then reappear some hours or even days later without any adjustment. Is it a TV, arial or transmittor problem. any suggestions